TY  - JOUR
AU  - Khadjesari, Zarnie
AU  - Murray, Elizabeth
AU  - Kalaitzaki, Eleftheria
AU  - White, Ian R
AU  - McCambridge, Jim
AU  - Thompson, Simon G
AU  - Wallace, Paul
AU  - Godfrey, Christine
PY  - 2011
DA  - 2011/03/02
TI  - Impact and Costs of Incentives to Reduce Attrition in Online Trials: Two Randomized Controlled Trials
JO  - J Med Internet Res
SP  - e26
VL  - 13
IS  - 1
KW  - Nonresponse
KW  - attrition
KW  - Internet
KW  - alcohol drinking
KW  - randomized controlled trial
AB  - Background: Attrition from follow-up is a major methodological challenge in randomized trials. Incentives are known to improve response rates in cross-sectional postal and online surveys, yet few studies have investigated whether they can reduce attrition from follow-up in online trials, which are particularly vulnerable to low follow-up rates. Objectives: Our objective was to determine the impact of incentives on follow-up rates in an online trial. Methods: Two randomized controlled trials were embedded in a large online trial of a Web-based intervention to reduce alcohol consumption (the Down Your Drink randomized controlled trial, DYD-RCT). Participants were those in the DYD pilot trial eligible for 3-month follow-up (study 1) and those eligible for 12-month follow-up in the DYD main trial (study 2). Participants in both studies were randomly allocated to receive an offer of an incentive or to receive no offer of an incentive. In study 1, participants in the incentive arm were randomly offered a £5 Amazon.co.uk gift voucher, a £5 charity donation to Cancer Research UK, or entry in a prize draw for £250. In study 2, participants in the incentive arm were offered a £10 Amazon.co.uk gift voucher. The primary outcome was the proportion of participants who completed follow-up questionnaires in the incentive arm(s) compared with the no incentive arm. Results: In study 1 (n = 1226), there was no significant difference in response rates between those participants offered an incentive (175/615, 29%) and those with no offer (162/611, 27%) (difference = 2%, 95% confidence interval [CI] –3% to 7%). There was no significant difference in response rates among the three different incentives offered. In study 2 (n = 2591), response rates were 9% higher in the group offered an incentive (476/1296, 37%) than in the group not offered an incentive (364/1295, 28%) (difference = 9%, 95% CI 5% to 12%, P < .001). The incremental cost per extra successful follow-up in the incentive arm was £110 in study 1 and £52 in study 2. Conclusion: Whereas an offer of a £10 Amazon.co.uk gift voucher can increase follow-up rates in online trials, an offer of a lower incentive may not. The marginal costs involved require careful consideration. Trial registration: ISRCTN31070347; http://www.controlled-trials.com/ISRCTN31070347 (Archived by WebCite at http://www.webcitation.org/5wgr5pl3s) 
SN  - 1438-8871
UR  - http://www.jmir.org/2011/1/e26/
UR  - https://doi.org/10.2196/jmir.1523
UR  - http://www.ncbi.nlm.nih.gov/pubmed/21371988
DO  - 10.2196/jmir.1523
ID  - info:doi/10.2196/jmir.1523
ER  -